Further progress is expected in lithium ion batteries for automobile use and for power storage use, and there is a growing demand for a low-temperature property in addition to a low resistance and a long product life. A conductive assistant is gaining importance as a material essential for attaining improvement of the battery properties. Examples of a major conductive assistant include carbon nanofiber, multi-layer carbon nanotubes and carbon black.
Carbon nanofiber is rigid carbon fiber having a relatively thick fiber diameter of 50 nm to 300 nm and a fiber length of about 10 μm. Such carbon nanofibers show weak entanglement between the fibers, and thus each of the carbon nanofibers can be easily dispersed by adding to a matrix and kneading. In addition, active substances can be easily connected to each other due to the very long carbon length of the carbon nanofiber. However, when trying to sufficiently construct electrically conductive networks by connecting the carbon nanofibers to each other, a large amount of the carbon nanofibers needs to be added.
On the other hand, carbon nanotubes have a thin fiber diameter of 5 nm to 40 nm and a fiber length of about 3 μm, showing an aspect ratio of several hundreds. Therefore, if they can be dispersed in a matrix, such an effect of improving the conductivity can be expected. However, the fibers of such carbon nanotubes are generally entangled each other to form aggregates of several hundred micrometers. When the aggregates of such strongly entangled carbon nanotubes is added to a matrix and kneaded, the aggregates only become finer, and the structure of the aggregates still remains. Therefore, it is difficult to achieve a state in which each of the carbon nanotubes is untangled. As a result, the carbon nanotubes sometimes have a little effect on imparting electric conductivity for its amount to be added.
Further, carbon blacks represent particles having a primary particle diameter of several nanometers to several tens of nanometers. The carbon blacks form a secondary structure called “STRUCTURE” in which primary particles are connected each other. The carbon black having a large specific surface area is excellent in a liquid retention property, and as a result, high input-output characteristics or an improvement effect can be expected. However, the STRUCTURE in this carbon black usually has a connection length of several hundreds of nanometers at most, and therefore the carbon black does not have a satisfactory cycle life property.
In order to compensate for the disadvantages and to utilize the advantages of each of the carbon nanofibers, the carbon nanotubes and the carbon black, studies have been made on a combined use of these materials as a conductive assistant.
In Japanese Patent Publication No. 4835881 (US 2012/0171566 A1; Patent Literature 1), a synergetic effect to reduce electric resistance by using carbon nanofibers and carbon nanotubes, and carbon nanotubes and carbon black in combination has been confirmed. However, satisfactory effect in terms of battery properties at a low temperature has not been attained and therefore further improvement is required.
Japanese Patent Publication No. 5497220 (US 2014-272596 A1; Patent Literature 2) discloses a method as described below as a method for obtaining composite carbon fiber comprising multi-walled carbon nanotubes, graphitized carbon nanofibers and carbon black particles. First, the carbon materials are each added to pure water and mixed to obtain a mixed liquid. The mixed liquid is separated into the carbon materials and pure water when allowed to stand still for several minutes. This shows that no physical change has occurred to the carbon materials. Subsequently, the mixed liquid is introduced with pressure using a pump into a grinding nozzle of a high-pressure dispersing device to obtain paste or slurry. As the mixed liquid passes through the nozzle at ultrahigh speed, strong shear force is generated by turbulence. By the shear force and the cavitation effect, the multi-walled carbon nanotube aggregates are untangled and are homogenously compounded with the graphitized carbon nanofibers and the carbon black particles.
Next, the resulting paste or slurry is dried for powdering. Examples of drying method include spray drying, lyophilization, drum drying, flash drying, hot-air drying, vacuum drying and the like.
The thus-obtained composite carbon fiber has a special structure in which carbon fibers, multi-walled carbon nanotubes and carbon black particles are homogeneously dispersed. However, even if this method is employed, the aggregation activity of the multi-walled carbon nanotubes in dry condition is enhanced when the mass ratio of the multi-walled carbon nanotubes in the composite carbon fiber exceeds a certain value, and it becomes difficult to redisperse the multi-walled carbon nanotubes when they are added to the matrix.
When multi-walled carbon nanotubes and graphitized carbon nanofibers are compared in terms of the aggregation activity, multi-walled carbon nanotubes having a smaller carbon diameter and a higher aspect ratio generally exhibit higher aggregation activity. In addition, in the case of the multi-walled carbon nanotubes produced by a supported catalyst method, they are entangled to each other like fuzzballs and more energy is required to untangle them and to produce a dispersion containing no aggregates. Therefore, in the case of performing a dispersing operation by adding multi-walled carbon nanotubes and graphitized carbon nanofibers to a solvent at the same time, excessive fracture of graphitized carbon nanofibers occurs under the condition suitable for the dispersion of multi-walled carbon nanotubes. In contrast, multi-walled carbon nanotubes will not be sufficiently dispersed under the condition suitable for the dispersion of graphitized carbon nanofibers.